A fundamental problem in the operation of cryogenic distillation processes for the separation of an acid gas-containing hydrocarbon feed stream is the formation of free-water and/or solid hydrates in the cryogenic distillation column. For example, in the cryogenic distillation process known as the Ryan/Holmes process (see U.S. Pat. No. 4,462,814 issued July 31, 1984, hereby incorporated by reference) a build-up of water may occur in the ethane or propane recovery distillation columns of the system.
In the normal cryogenic distillation processing, for example with the Ryan/Holmes process, the water entering the Ryan/Holmes process with the feed gas must exit with the bottom product. The amount of water which is able to leave by the bottom product is related to the total acid gas content of the bottom product, such as the carbon dioxide and hydrogen sulfide content, with the result that either a low feed gas water concentration of less than 5 ppm, or a high bottoms acid gas content of greater than 0.1 mole percent, must be employed. A low feed gas water concentration requires specialized techniques to be employed other than the conventional tetraethylene glycol (TEG) dehydrating, such as the use of "cold-finger" or stripping gas processes or even molecular sieve or other specialized dehydration. High bottoms acid gas content requires additional downstream acid gas removal processing which is also undesirable.
In the cryogenic distillation systems employing a hydrocarbon feed gas, typically the temperatures at or near the top of a column provide for high liquid phase water concentration. The water vapor is absorbed into the descending liquid phase and enters the lower part of the column. The cryogenic distillation column also generally employs a reboiler and typically the temperatures in the bottom section are sufficiently high to force the water back into the vapor phase. Thus, in the cryogenic distillation system, the water cannot go out either with the overhead stream or the bottom stream, but builds up to form a separate liquid free-water layer or solid hydrates within the cryogenic distillation column. In order to solve this problem, some cryogenic distillation columns have a water draw off tray in about the middle of the column for decanting the water, and the water draw off tray requires a periodical drawing off of liquid water.
However, in the presence of acid gases in the distillation column, such as carbon dioxide and hydrogen sulfide, the presence of water leads to a corrosive environment, so that materials of construction of the column must be substantially upgraded to prevent corrosion. In some columns, the presence of water could form solid hydrates, such as for example, water carbon dioxide or water hydrocarbon hydrates. Water hydrates can be formed with hydrocarbon feed streams in columns containing temperatures as high as up to 60.degree. F., but typically the problem is more acute at temperatures of 50.degree. F. or below. Thus, if water is not removed from the cryogenic distillation column, it forms a liquid water phase on the distillation tray above the distillation tray where the maximum water vapor concentration occurs, and therefore, leads to the use of a water draw off tray to remove periodically the liquid water.
It is therefore desirable to provide a simple, effective and efficient system to prevent free-water formation in a cryogenic distillation column.